Placing everyting witin a DIV

I want to place everything within a DIV so that I can center it and then give it a background. If you look at stylebuffet.com/forums, you’ll see I have kind of got it, but I want the white to go all the way uo nd down like it does here on the BBpress forums. Can anyone help?

Like this guy did here where there is the grey background and then everything else is in white